1. Field of the Invention
The present invention relates to a plasma processing technology, and more particularly to a technology for controlling plasma potential and substrate potential, adaptable advantageously, for example, to a sputtering apparatus for forming a Si, Al or SiO.sub.2 film on a substrate in the preparation of semiconductor devices.
2. Related Background Art
Sputtering has been employed as a method for forming a thin film on a substrate such as a semiconductor wafer. In the ordinary sputtering method, in a chamber that can be evacuated (vacuum chamber), there is generated a vacuum state of about 10.sup.-3 to 10.sup.-2 Torr, and a DC or high frequency power is applied to a first electrode, supporting a target, to generate plasma discharge, thereby accelerating cations to and bombarding the first electrode constituting a cathode by the cathode voltage drop. Atoms released by the impact from the target material provided on said first electrode are deposited on a substrate placed in the vacuum chamber and form a thin film thereon.
In a biased sputtering method, which is recently attracting attention, a DC or high frequency power is applied also to a second electrode supporting the substrate and positioned opposite to the first electrode receiving the plasma generating power, whereby the cations are also accelerated toward and bombard the substrate during film formation.
Alternatively, there is also proposed, for example in the Japanese Laid-open Patents Sho 59-104111 and Sho 61-231172, a method of positioning a third electrode in the vicinity of the substrate or the target and effecting film formation while a DC voltage is applied between said third electrode and the ground.
With the continuing progress in the performance and level of integration of semiconductor devices, the requirements for the quality of films obtained by film forming methods such as sputtering for use in such semiconductor devices have become more and more stringent.
Conventional sputtering methods are not necessarily capable of satisfactorily responding to such requirements.
For example in the ordinary sputtering method, the plasma potential is determined by the plasma discharge conditions (discharge pressure, electric power, electrode size etc.), and it is difficult to control the plasma potential at an arbitrary value independently from the plasma discharge conditions. Also the substrate potential may be floating with respect to the ground potential or the plasma, depending on the material of the substrate and the state of positioning thereof, so that the controllability of substrate potential is also poor. Consequently the energy of cations entering the substrate at the film formation (corresponding to the difference between the plasma potential and the substrate potential) is difficult to control, and this fact may lead to deterioration of the film quality. Also the plasma potential may be significantly elevated under certain plasma discharge conditions, showing a large potential difference to the vacuum chamber or a deposition preventing plate, with which the plasma comes into contact. Consequently the cations in the plasma hit such vacuum chamber or deposition preventing plate, and the atoms emitted therefrom tend to enter the thin film formed on the substrate. For this reason, there is inevitably a limit in the reduction of contamination in the resulting thin film, resulting from materials from such vacuum chamber or deposition preventing plate.
On the other hand, the aforementioned biased sputtering is capable of forming a thin film with impact of cations of a larger energy than in the ordinary sputtering, by the control of the substrate potential, thereby providing a thin film of an improved mechanical strength. Also, this biased sputtering can provide a film of flat surface, utilizing a fact that a film formed on a stepped portion is easily subjected to sputter etching. This method, however, is associated with a drawback of a very small film forming speed, since the sputter etching is conducted simultaneously with the film formation. Also it may damage the substrate, since the film formation is conducted with direct bombardment of the substrate with cations of a larger energy than in the ordinary sputtering. Also as the controllability of the plasma potential is as poor as in the ordinary sputtering, the cations in the plasma tend to hit the vacuum chamber or the deposition preventing plate, whereby the atoms emitted therefrom tend to enter the thin film formed on the substrate, and there is inevitably a limit in reducing the migration of such contaminates into the film.
In the sputtering method disclosed in the aforementioned Japanese Laid-open Patent Sho 59-104111, a rod-shaped third electrode opposed to the substrate is given a positive voltage to elevate the plasma potential, whereby the film formation is conducted with a substantial negative bias voltage applied to a part of the substrate. This method can provide a thin film for example with improved magnetic characteristics. However a positive voltage applied to the third electrode elevates the plasma potential, thus increasing the difference between the plasma potential and the substrate potential and eventually causing, in certain cases, bombardment of the substrate with cations of a larger energy than in the method without the third electrode. In such case the damage to the substrate becomes more apparent. Also the elevation of the plasma potential further increases the difference between the plasma potential and the vacuum chamber or the deposition preventing plate, and the atoms emitted from such members by the impact of cations easily enter the thin film formed on the substrate, so that a film with a relatively high contamination with the materials from such vacuum chamber or deposition preventing plate is often obtained.
Also in the sputtering method disclosed in the aforementioned Japanese Laid-open Patent Sho 61-231172, a third electrode is positioned in the vicinity of the target, and the potential of said third electrode is controlled to vary the plasma potential, whereby the film formation on the substrate is conducted under controlled current into the substrate. The object of this method is to achieve film formation by sputtering with reduced damage.
However, if the substrate or the film to be formed thereon is an insulating material, the current flowing into the substrate is zero even with the voltage application to the third electrode, since the ion current becomes equal to the electron current. It is therefore not possible to control the current into the substrate by the potential control on the third electrode, so that the prevention of damage of the substrate cannot be achieved by the voltage application to the third electrode.
Also in case of a positive voltage application to the third electrode, the plasma potential is elevated to increase the potential difference between the plasma potential and the vacuum chamber or deposition preventing plate, thereby resulting in the drawback, as in the aforementioned sputting methods, of contamination of the thin film formed on the substrate, by the atoms emitted from such vacuum chamber or deposition preventing plate by the bombardment of the accelerated cations.
As explained in the foregoing, the conventional sputtering methods have been associated with a limit in the reduction of contamination from the vacuum chamber, deposition preventing plate etc., and possible damages in the substrate.
Consequently the conventional sputtering methods are often not adequate for the manufacture of products of high quality.